Manual Calibration

For images where the beam centre is off the detector, there are few complete rings or there is a high tilt, try Manual Calibration (requires starting guess for energy and experimental geometry)

Scripting Automatic Calibration in Jython

For images which contain the beam centre and multiple complete rings and have a detector tilt of less than 10 degrees - calibration can be scripted in Jython.

Theory

There are two different methods used in DAWN to calibrate powder diffraction images, Ellipse Parameter Calibration and Point Parameter Calibration. Here is a quick introduction to the calibration routine and the differences between the two methods.

Finding points of interest (POI)

The first step in the calibration process is to find the location of points of interest marking where the diffraction rings from the standard are in the image. DAWN takes a line profile across the ring, at many points around the ring and the peak fits the profile to find the maximum. The position of the maximum, transformed back into the image co-ordinates is a point of interest. The difference between the Ellipse Parameter Calibration and Point Parameter Calibration is what is done with these POIs to calibrate the experiment.

The Ellipse Parameter Calibration works best using multiple images at a very well defined separation and requires no indication of the energy or distance the measurement was taken at. The beam centre and detector tilts are explicitly calculated during this calibration procedure, and so cannot be fixed.

Point Parameter Calibration

The Point Parameter Calibration is a more basic approach than the Ellipse Parameter Calibration and simply relies on the fact that each POI found on a standard ring corresponds to a specific q value, and that doing an initial "by-eye" alignment gives a close energy/wavelength and experimental geometry.

From these initial energy and geometry parameters, q values for each point of interest can be calculated. During the calibration, the energy, distance, beam centre and detector tilts are adjusted until the model POI q values best match the expected standard q values.

Since this process does not rely on the accuracy of an ellipse fit, the Point Parameter Calibration can be used for partial rings or very high tilts, but has the dis-advantage of needing a good set of initial parameters. Since all parameters are determined by fitting any can be fixed, for example if the energy and beam centre have already been determined.